1. Field of the Invention
The invention concerns a unit for the dosage of grained, pourable materials, in particular blasting abrasives, for a processing of workpieces, large surfaces or the like.
2. Description of Related Art
A unit of the above mentioned kind is known from the EP-A-0 578 132. An abrasive feeding device is arranged beneath a vessel which is filled with abrasive. The feeding device consists of a tube in which a dosage auger is installed. The abrasive transported by the auger falls through a tube arranged at the end of an outfeed area. At the tube there is a flow measuring device which transfers received measuring signals to an evaluation unit which evaluates the measuring signals and compares the result with nominal values. The rotation speed of the dosage auger is set by means of these results.
It is disadvantageous that although the metering screw has a double-lead screw, not all granular flowable materials can be processed. Even when the dosage auger has double spiral wings, this is not sufficient for the use of all grained, pourable materials for the treatment process. Very fine materials behave similar to flour. The double spiral wings of the dosage auger with its tapering end does not operate well with a uniform dosage, but presses the flour like materials together so that the abrasive passes in lumps through the downpipe. Thus, the function of the flow measuring device and the evaluation unit is severely disturbed. The flow measuring device determines highest and lowest material densities so that the evaluation unit sets the dosage auger to zero or to a maximum speed.
Furthermore, the transport of the materials coming out of the downpipe to the blast nozzle has to be improved. Since the arriving materials are only blown away, a single and directed effect of each grain is not achieved.
U.S. Pat. No. 2,536,250 discloses a cleaning unit where a mixing link is arranged beneath an abrasive feeding device of an abrasive collecting hopper. The mixing link consists of a cross and a longitudinal boring which are connected to a mixing chamber. In this boring a nipple is mounted which is arranged with its tip behind the connecting point of the cross boring, i.e., outside the mixing chamber, and thus ends before an adaptor. A line with a relatively big diameter is connected to the adapter.
It is a disadvantage that the compressed air streaming into the mixing chamber only creates a vacuum towards the abrasive flowing into the mixing chamber. The abrasive is aspirated by the vacuum and pressed into the line. For the support of the transport capacity of the mixing chamber, the longitudinal boring must be directed downwards since the transport pressure on the line is so small that the line length and thus the effectivity of the abrasive is limited. In order to achieve the abrasive's cleaning effect at least steam must be added.
GB-A-182,628 discloses a shot peening device which includes a dosage auger with one inlet which is fed with granulate media by a hopper. The dosage auger is driven by a step motor in order to advance the media fed into the infeed with an exactly controlled speed to an outfeed line. The motor is controlled by a computer so that a programmed quantity comes out of the outfeed line. A capacitive proximity switch is provided in the outfeed line in order to determine whether the outfeed is blocked.
It is also not possible to use all materials, even those which are not similar to flour, without malfunctions in this unit. Furthermore, the materials are only blown away after leaving the dosage auger, so that the single grain cannot achieve its full efficiency which, however, is absolutely necessary for shot peening processes.
EP-A-0 218869 discloses a unit for the uniform dosage of grained abrasive for pneumatically operating blast machines. This unit stores abrasive in a closed vessel. A dosage auger is arranged beneath an outlet of the vessel. The dosage auger turns in a horizontal tube. Thereby, the infeed area of the dosage auger receives abrasive material and feeds it to the outfeed area. At one end the auger tube is connected to ducting in which the abrasive is fed. Since the ducting is pressurized the abrasive is carried along and directed to the blast nozzle. To assure that the abrasive being fed is uniform, a unit is provided which balances the pressure drop from the inside of the closed vessel to the inside of the auger tube up to the feeding line.
However, in order to be able to process all materials and to achieve its full efficiency, the present means to render the material uniform are not sufficient.
Thus, there is the task to continue the development of a unit for the dosage of grained, pourable materials, in particular abrasives, so that all material configurations can be used to full efficiency.
This task is solved by the features of the present invention.